This invention relates generally to a voltage regulator, and more particularly to a voltage regulator including a thermally activated switch.
Voltage regulators are used to limit the voltage level supplied by an energy source to a load. For example, a voltage regulator is used to limit the voltage to which a battery is charged by a source such as a photovoltaic array. Without the regulator, the battery could be overcharged and damaged.
Regulators are generally classified as either series type or shunt types with the name derived from whether the active element of the regulator is in series with the source or in parallel with the source. The active element of the regulator may be either a linear or a switching device. In the former, a suitable feedback circuit controls the active element which regulates the output of the source to the load and limits the output to a controlled range. For example, in charging a battery, the linear regulator regulates the output to the battery and keeps the battery at a "float" voltage. The active device in a switching regulator is either fully off or fully on and has the advantage, over the linear regulator, of significantly lower power dissipation. A switching regulator is thus usually preferred.
A shunt regulator usually has a power dissipation advantage over a series regulator because the active series element of a series regulator dissipates power at all times power is delivered to the load. This disadvantage of a series regulator, however, essentially disappears if the series element has a low resistance, such as is characteristic of a simple switch. A series regulator is preferred in some applications because such a regulator does not require shorting out the source as does a shunt regulator. Shorting out the source can be a significant problem with photovoltaic arrays, especially at high voltages, and may be prohibitive with other power sources. For the foregoing reasons, voltage regulators are preferably of the switching series type with a low impedance active element in series with the source.
One type of series element used in this type of regulator is an electromechanical relay. The electromechanical relay has the low impedance required of the switching element, in comparison to, for example, semiconductor devices, but electromechanical relays have a very limited lifetime. Such relays are typically limited to 10.sup.5 -10.sup.6 switch cycles. In a battery charging application, to insure that the electromechanical switching element lasts as long as the ten year expected lifetime of the battery, the relay must therefore be limited to an average switching rate of 1 per 2 hours to 1 per 10 minutes. In a typical battery charging application, a control circuit removes the battery charging current when the voltage at the battery reaches a predetermined overvoltage level. The battery again starts charging when the battery voltage drops to some lower voltage value. This type of cycle requires one opening and one closing of the switching element. A fully charged battery, however, will cycle under these conditions in a matter of seconds. To prolong the lifetime of the regulator, additional circuitry must be used to reduce the switching rate of the electromechanical relay. The additional circuitry, of course, adds to the cost of the regulator and adversely affects the reliability of the regulator.
A need therefore existed for a regulator which would overcome the foregoing problems to provide an inexpensive and reliable regulator.
It is therefore an object of this invention to provide an improved thermally activated voltage regulator.
It is a further object of this invention to provide an improved voltage regulator for use with a photovoltaic array.